At present, in the field of wireless network, the Wireless Local Area Network (WLAN) develops rapidly, and the application range of the WLAN is increasing. In order to meet various network demands, the Institute of Electrical and Electronic Engineers (IEEE) 802.11 group has issued a series of WLAN technology standards such as 802.11a, 802.11b, 802.11g, 802.11n etc., and subsequently, other task groups are set up, which are committed to develop and design specifications of improvements of the existing 802.11 technology. For example, with the development of the Internet of things, the 802.11 ah task group is set up by the IEEE, and its main task is to modify and enhance a Medium Access Control (MAC) layer and a Physical Layer (PHY) of the WLAN, so as to adapt to demands for networks such as a smart grid, a sensor network, environmental/agricultural monitoring, industrial process automation etc.
In the wireless LAN, an Access Point (AP for short) and multiple Non-AP Stations (STAs for short) associated with the AP constitute a Basic Service Set (BSS for short). Before using the BSS service, the STA must complete authentication and association process with the AP. If the association is successful, the AP allocates an Association Identifier (AID for short) to the STAs. The AID is an identity of the STA in the BSS, i.e., the STA can be distinguished from other STAs in the BSS through the AID, but STAs in different BSSs may use the same AID. At the same time, after multiple BSSs are connected by a Distribution System (DS for short), an Extended Service Set (ESS for short) can be constituted. The multiple STAs can also constitute a self-organization WLAN, which is referred to as an Independent BSS (IBSS). The STAs in the IBSS can communicate with each other directly.
In the 802.11, an Access Point (AP for short) and multiple Stations (STAs for short) associated with the AP constitute a Basic Service Set (BSS for short). The 802.11 defines two operating modes: Distributed Coordination Function (DCF for short) and Point Coordination Function (PCF for short), and the improvements for the two operating modes: Enhanced Distributed Channel Access (EDCA for short) and Hybrid Coordination Function Controlled Channel Access (HCCA for short). Wherein, the DCF is the most basic operating mode, which enables multiple STAs to share a wireless channel using a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism. The EDCA is an enhanced operating mode, which maps the upper layer data into four different Access Categories (ACs): AC_VO, AC_VI, AC_BE and AC_BK, wherein, each access category distinguishes between priorities using parameters of different competition channels. With the CSMA/CA mechanism, the EDCA enables accesses with different priorities to share a wireless channel, and reserves a Transmission Opportunity (TXOP for short). A basic process of accesses with different priorities obtaining transmission opportunities is that when each access is idle in the channel, the channel is accessed using different backoff intervals plus any competition backoff window, to reduce collision.
When the multiple wireless STAs share a channel, it becomes very difficult to detect collision in a wireless environment, in which a serious problem is hidden STAs. As shown in FIG. 1, STA A transmits data to STA B, and STA C also transmits data to STA B at the same time. Because the STA C and the STA A are within a coverage of each other, simultaneous transmission of the STA A and the STA C will result in conflict. From the perspective of the STA A, the STA C is a hidden STA. In order to solve the problem of the hidden STA, the 802.11 proposes a virtual channel detection mechanism, i.e., collision with the hidden STA is avoided by means of including channel reservation time information in a frame header of a radio frame. Other STAs receiving the radio frame with reservation time information set a Network Allocation Vector (NAV) stored locally. A value of the NAV is set as a maximum value of the above channel reservation time information and reserved time information. In this period of time, the other STA will not transmit data, thereby avoiding the problem of the hidden STA competing for a channel which results in collision. Only after the NAV is reduced to zero, other STAs will transmit data. Before large data are transmitted, a transmitting party can firstly transmit a Request to send (RTS) for channel reservation, which includes channel reservation time information. A receiving party responds with Clear to send (CTS) to make a channel reservation acknowledge, which also includes channel reservation time information, to protect radio frames transmitted subsequently by the transmitting party, and FIG. 2 can be referred to for a specific process.
The IEEE 802.11 defines two modes of energy management: an Active Mode (AM) and a Power Saving Mode (PS). For an STA in a power saving mode, the AP carries traffic indication information in a Beacon transmitted periodically, i.e., a Traffic Indication Map (TIM for short) information element, which notifies whether these STAs in a power saving mode have bufferable units to be transmitted. If it finds that these STAs have bufferable units, the STAs will transmit a Power Saving frame (PS-Poll), and the AP can respond with an Acknowledgement (ACK) frame or can also directly respond with a data frame. An STA which sleeps for a long time awakes, and actively transmits the PS-Poll frame, and the AP responds by transmitting an ACK carrying an indication of whether there is downlink traffic or directly transmitting a data frame.
For an application scenario with a large coverage and multiple STAs, the number of STAs which can not be listened for each other will increase, and the problem of the hidden STAs will become more obvious. If a frame responded by an AP is long after the STA transmits the PS-Poll frame, the time for transmitting the frame will be long. At the STA side, an STA which can not receive the response from the AP will judge that the channel is empty, compete for a channel and transmit data, which results in collision at the STA side which transmits the PS-Poll.